We use steady-state and time-resolved spectroscopy to evaluate optoelectronic material quality and obtain detailed information about carrier generation, transport, and relaxation in semiconductor devices and test structures. This report focuses on time-resolved and steady-state photoluminescence of III-V reference heterostructures at temperatures between 4K and 300K in order to investigate the mechanisms limiting carrier lifetime and to develop the capability to provide actionable feedback to research-and-development efforts for improvement and optimization of material properties and/or device performance. We combine the results of photoluminescence experiments with model-based analyses and simulations of carrier relaxation to assess the impacts of defects and interface quality on the relaxation dynamics of photo-generated carriers in double heterostructure test vehicles grown by MOCVD and MBE.
III-V multi-junction solar cells are typically based on a triple-junction design that consists of an InGaP top junction, a GaAs middle junction, and a bottom junction that employs a 1 – 1.25 eV material grown on GaAs substrates. The most promising 1 – 1.25 eV material that is currently under extensive investigation is bulk dilute nitride such as (In)GaAsNSb lattice matched to GaAs substrates. The approach utilizing dilute nitrides has a great potential to achieve high performance triple-junction solar cells as recently demonstrated by Wiemer, et al., who achieved a record efficiency of 43.5% from multi-junction solar cells including MBE-grown dilute nitride materials [1]. Although MOVPE is a preferred technique over MBE for III-V multi-junction solar cell manufacturing, MOVPEgrown dilute nitride research is at its infancy compared to MBE-grown dilute nitride. In particular, carrier dynamics studies are indispensible in the optimization of MOVPE materials growth parameters to obtain improved solar cell performance. For the present study, we employed time-resolved photoluminescence (TR-PL) techniques to study carrier dynamics in MOVPE-grown bulk dilute nitride InGaAsN materials (Eg = 1 – 1.25 eV at RT) lattice matched to GaAs substrates. In contrast to our earlier samples that showed high background C doping densities, our current samples grown using different metalorganic precursors at higher growth temperatures showed a significantly reduced background doping density of ~ 1017 /cm3. We studied carrier dynamics in (In)GaAsNSb double heterostructures (DH) with different N compositions at room temperature. Post-growth annealing yielded significant improvements in carrier lifetimes of (In)GaAsNSb double heterostructure (DH) samples. Carrier dynamics at various temperatures between 10 K and RT were also studied from (In)GaAsNSb DH samples including those samples grown on different orientation substrates.
The ability to follow and observe single molecules as they function in live cells represents a major milestone
for molecular-cellular biology. Here we present a tracking microscope that is able to track quantum dots in
three dimensions and simultaneously record time-resolved emission statistics from a single dot. This innovative
microscopy approach is based on four spatial filters and closed loop feedback to constantly keep a single quantum
dot in the focal spot. Using this microscope, we demonstrate the ability to follow quantum dot labeled IgE
antibodies bound to FcεRI membrane receptors in live RBL-2H3 cells. The results are consistent with prior
studies of two dimensional membrane diffusion (Andrews et al., Nat. Cell Biol., 10, 955, 2008). In addition, the
microscope captures motion in the axial (Z) direction, which permits tracking of diffusing receptors relative to
the "hills and valleys" of the dynamically changing membrane landscape. This approach is uniquely capable of
following single molecule dynamics on live cells with three dimensional spatial resolution.
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